Solar resources are the fuel for solar energy technologies. Information on solar resources, combined with information on energy prices as well as investment costs, forms the basis of the economic viability of any solar energy project.
For solar prospection and pre-feasibility studies, information in the form of maps is used. Maps play a significant role in regional inventories and in defining solar energy strategies.
For Africa, such maps are available from at least four sources: NASA, NREL/SWERA, PVGIS/HelioClim-1 and SolarGIS. The information value of the maps can be judged by comparing their spatial resolution and the accuracy of the underlying solar radiation data.
Low and medium-resolution maps provide an overview of solar potential and these maps have been available in the public domain for years now, for example in the Solar and Wind Energy Resource Assessment (SWERA) and Photovoltaic Geographical Information System (PVGIS) online platforms. However, they do not meet the requirements for a detailed understanding of solar potential and cannot be used for solar power plant site selection.
Detailed solar maps were unavailable until the problem was addressed by investment in solar resource assessment and progress in solar resource modeling, as well as the implementation of innovative IT and cartographic solutions. As a result, high-resolution data showing long-term climate records have been made available to key players such as policymakers, developers and investors.
The detailed digital maps are accessible via interactive map applications, such as SolarGIS iMaps or Solar Atlas for the Mediterranean. They are also used as data layers in geographical information system (GIS) inventories. The need for high quality and detailed maps has also been expressed by international initiatives, such as the Global Atlas of Wind and Solar Resources by the International Renewable Energy Agency and ESMAP by the World Bank.
African solar resource mapping
Several public agencies have developed solar resource data and maps covering, among other regions, Africa. Geographical coverage, length of climate record, accuracy and data resolution were determined by atmospheric and meteorological data from satellites, IT, communication technology and other resources.
NASAs maps are well known. The NASA Surface meteorology and Solar Energy (SSE) project was launched in 1997 and its purpose is to provide a global overview of solar resources from 1983 to 2005. Because the database has a very low resolution of approximately 110 km (at the equator), it is neither suitable for site selection nor bankable solar resource assessment. Figure 2 below shows that reliance on the NASA SSE data set may result in solar radiation estimates dramatically different from reality.
Another commonly used database was developed by the National Renewable Energy Laboratory (NREL) from 2001 to 2005 as part of the United Nations Environment Programs SWERA project.
For Africa, the NREL data represent 1985 to 1991 with a spatial resolution of 40 km, providing information in about nine times higher detail than NASA. Information on the data accuracy is limited. In addition, figure 2 demonstrates that in this case also, spatial variability of solar resources can result in a choice of location that does not have maximal solar potential.
In 2005, the European Commissions in-house scientific service, the Joint Research Centre (JRC), extended its PVGIS online application from Europe to Africa. This was possible due to a collaboration between JRC and MINES ParisTech/ARMINES.
The HelioClim-1 database, with an original resolution of approximately 30 km, was enhanced to 2 km and the monthly averaged data were made available through PVGIS for 1985 to 2005. Besides solar data, PVGIS provides simple PV electricity simulation tools. In 2010 more accurate satellite-based data from CM-SAF were integrated into PVGIS, representing 1998 to 2012 and PVGIS was established as a research and policy support tool.
Because of its simplicity and easy access, it has gained the confidence of the solar industry over the last decade. In the period of its main development it represented state-of-the-art concepts but it does not fulfil current needs for bankable information.
Solar resource map requirements
What databases made by NASA, NREL and JRC (PVGIS) have in common is that each significantly contributed to the improvement of solar maps and extended public awareness. However, today commercial databases receive significantly more attention as the solar industry identifies new needs for accurate data and customized services.
Growing interest in solar energy in African countries has triggered a need for detailed regional maps, used for defining national solar energy plans and for selection of candidate sites. Besides static solar maps, solar energy strategies should include plans for establishing country-wide solar data infrastructure, including satellite-based maps and a network of ground radiometers. As a result solar data are moving beyond their historical role of providing static map information as annual or monthly long-term averages. National solar maps are used for site selection, for which site-specific data products are needed, such as hourly or sub-hourly multi-year time series or Typical Meteorological Year (TMY) data.
An important feature of modern data is continuity, which is essentially ensuring the same methods are applied for the computation of the historical data used in planning as well as for data computed in real-time for monitoring and forecasting. Combining long histories of satellite data with short campaigns of ground-measurements increases the accuracy of solar radiation data.
Correlation of satellite data and ground measurements is a basic precondition for the bankable assessment of large solar power plants.
Commercial sources of solar data
One of the first commercial sources for solar resource data is Meteonorm, managed by Meteotest. Global coverage, compactness and a long position on the market made this product very popular. On the other hand, Meteonorm is not a typical database but rather a piece of software for the interpolation of ground-measured solar and meteo data.
HelioClim-3, developed by MINES ParisTech and operated by Transvalor, is a satellite-based database with a long history, where data and maps are offered via the SoDa online portal. A complete coverage of Africa is also offered by the 3TIER company, via their FirstLook portal and on request.
Other suppliers as well offer on request satellite-based or modelled data for selected sites or regions, for example, DLR, IrSOLaV, the University of Oldenburg, S2M, and so on.
The newest satellite-based database, SolarGIS, has been developed by GeoModel Solar (see figure 1, p. 81). Africa is represented by data from 1994 to the present, with a primary time resolution of 15 and 30 minutes. The original 3 km resolution has been enhanced to 250 metres in the high-resolution database and seamless interactive map application iMaps (see figure 2, pp. 80-81 and figure 3 above).
Most of the satellite data suppliers participated in the inter-comparison activity led by the University of Geneva in 2011. The resulting study recognized SolarGIS as the best solar database on the market in terms of accuracy and geographical coverage.
In addition, SolarGIS was acknowledged at the SNEC 2012 exhibition in Shanghai, China as one of the most advanced technologies in photovoltaics.